A Brake Pad Wear Control Algorithm for Electronic Brake System

Transcription

1 Advanced Materials Research Online: ISSN: , Vols , pp doi: / Trans Tech Publications, Switzerland A Brake Pad Wear Control Algorithm for Electronic Brake System Wei Li 1,a, Hongyu Zheng 1,b* and Changfu Zong 1,c 1 State Key Laboratory of Automotive Simulation and Control, Jilin University, Changchun , China a liweijlu@126.com, b zhenghy@jlu.edu.cn, c cfzong@yahoo.com.cn * Corresponding author: Hongyu Zheng Tel: ; zhenghy@jlu.edu.cn (Hongyu Zheng). Keywords: Electronic Brake System; Brake Pad Wear; Braking Force Distribution; Control Algorithm Abstract. A brake pad wear control algorithm used under uncritical braking conditions is proposed to reduce the difference in brake pad wear between the front and rear axles caused by the difference in braking force and the type of brake. The algorithm regulates the distribution of braking force within the limits of certain braking regulations according to the wear conditions of the brake pads while deceleration control still functions properly. Computer co-simulations of braking with Trucksim and Matlab/Simulink was performed in which vehicle models with equal brake pad wear, greater wear on the front axle and greater wear on the rear axle were used. The results show that the difference in brake pad wear between the front and rear axles can be reduced by distributing the braking force according to the wear conditions of the brakes when braking uncritically to reduce the time and cost needed in repair and maintenance. Introduction Pneumatic braking system has been widely used on commercial vehicles due to its reliability and its ability to generate large braking torques. Traditional pneumatic braking systems have long pneumatic lines which lead to longer response time. Therefore electronic brake system (EBS) was developed to improve braking performance. EBS controls the system through electronic signals, so vehicles equipped with EBS have a much shorter braking distance duo to shorter response time. EBS can distribute braking force dynamically and has integrated brake management functions such as Anti-lock Braking System (ABS), Acceleration Slip Regulation (ASR) and Electronic Stability Program (ESP) [1-3]. It is becoming popular on commercial vehicles in Europe and in the United States while it is still under study in China [4-7]. The wear condition of the brake pads depends greatly on the distribution of braking force and the type of brakes, so the wear conditions are usually different [8]. From the perspective of braking force distribution, 60% braking force is distributed to the rear axle on a bus, leading to greater wear on the rear axle. While from the perspective of brake types, the brake pads on the front axle will have greater wear if brake discs are applied to the front axle and drum brakes are applied to the rear axle. Different wear conditions make it impossible to replace all brake pads at the same time, which increases the time and cost needed in repair and maintenance. EBS from WABCO includes the function of brake pad wear control to regulate the braking pressure for each wheel according to the wear conditions. [1] Both the deceleration of vehicle and optimum distribution of braking force depend on load conditions. The deceleration control algorithm of EBS makes the total braking force proportional to the mass of the vehicle, which makes the braking performance independent from the load All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-21/02/16,08:32:03)

2 2100 Manufacturing Process and Equipment conditions when using the same pedal operation. Then the total braking force is distributed to each wheel based on the dynamic wheel load to make the adhesion utilization of each axle equal. So the optimum distribution of braking force is achieved at the same time. A brake pad wear control algorithm used under uncritical braking conditions is proposed in this paper to reduce the difference in brake pad wear conditions between the front and rear axles. The distribution of braking force is regulated within the limits of certain braking regulations according to the wear conditions of the brake pads while deceleration control still functions at the same time. Computer co-simulations of braking with Trucksim and Matlab/Simulink using vehicle models with equal brake pad wear, greater wear on the front axle and greater wear on the rear axle respectively will be performed, by which the efficiency of the algorithm will be verified. Braking Force Distribution Vehicles with traditional braking systems would spin out or lose steering capability because of wheel lockup caused by the fixed distribution of braking force between the front and rear axles when braking critically. Studies have shown that the optimum braking force distribution can be achieved when the adhesion utilization of each axle are equal to the braking rate z. [9-10] This will increase the braking rate at which wheel lock occurs and improve the braking stability by making full use of the road adhesion conditions. To achieve optimum braking force distribution, the braking force distributed to each wheel should be based on the dynamic axle loads, as described in Eq.1. φ =. (1) where is the adhesion utilized by axle i(i=1, front axle; i=2, second axle); is the braking force on axle i; is the normal reaction of road surface on axle i.[9] We propose a control algorithm of braking force distribution based on the dynamic axle loads [11]: the objective braking rate z is calculated according to brake pedal travel, then z is assigned to the objective adhesion utilization, z=. The objective braking force at each wheel is calculated by Eq.2. F =F *z. (2) Where is the objective braking force at wheel j; is the normal reaction of road surface on wheel j. The objective braking pressure for each brake actuator is determined according to the relationship between the braking pressure and the braking force exerted by the brake at each wheel. Deceleration Control The deceleration control makes the braking characteristics always constant under any load conditions, and it will be achieved when distributing the braking force according to the dynamic axle loads, as described in Eq.3. a=(f +F )/m=(f *φ +F *φ )/ m=z*g. (3) Where a is the deceleration of vehicle; m is the total mass of vehicle.

3 Advanced Materials Research Vols Brake Pad Wear Control The brake pad wear control intervenes when braking uncritically and detecting a difference in the pads between the front and rear axles. The pressure of the brakes with greater wear is released while the pressure of the brakes with lower wear is increased by a certain amount. Therefore the wear is regulated to make it possible to replace all the brake pads of the vehicle at the same time. The regulation of braking force is subject to three constraints: 1) the requirements of international automotive braking regulations; [12]2) the total braking force should only be determined by the mass of vehicle whatever the wear conditions are, so that the deceleration control can function properly; 3) to reduce the difference in brake pad wear conditions between the front and rear axles as soon as possible. To meet all these constraints, the brake pad wear control consists of three parts. 1) In part one, the objective braking rate z is firstly calculated according to brake pedal travel. Then the objective total braking force is derived as follows: F=m*z*g. (4) Where F is the total braking force; g is the acceleration due to gravity. 2) Part two determines the maximum braking force distributed to the wheels with lower wear within the limits of international automotive braking regulations (Or select an appropriate parameter within the limits of certain Regulations). In this paper, we take an algorithm for vehicles of category M 3 as an example. Mode one of the brake pad wear control is under the assumption that the brake pads of the front axle have greater wear. Then the objective adhesion utilization for the rear axle is determined by the objective braking rate z and braking regulations. The objective adhesion utilization for the rear axle is given by the equation φ =z+0.08 for braking rates between 0.15 and 0.3; φ =(z-0.02)/0.74 for braking rates z 0.3; to brake smoothly, φ =23/15*z for braking rates z<0.15. Mode two of the brake pad wear control is under the assumption that the brake pads of the rear axle have greater wear. Then the objective adhesion utilization for the front axle is determined by the objective braking rate z and braking regulations. The objective adhesion utilization for the front axle is given by the equation φ =(z+0.07)/0.85 for braking rates z 0.1; To brake smoothly, φ =0.2*z for braking rates z<0.1. Then the objective braking force at wheel j will be given by equation 2, =. 3) In part three, the braking force at wheels with greater wear is calculated by subtracting the braking force distributed to the wheels with lower wear from the total braking force. The brake pad wear control makes it possible to distribute braking force within the limits of certain braking regulations according to the wear conditions of the brake pads while the deceleration control is still available. The concept for all control algorithms is shown in Fig. 1. Fig. 1 Concept for all control algorithms

4 2102 Manufacturing Process and Equipment Controller Model The controller model built using Matlab / Simulink consists of functions such as deceleration control, optimum braking force distribution, brake pad wear control, etc. The inputs to the controller model are the states of load, the brake pad wear conditions and the speed of vehicle, the output are the braking pressures to the actuators. Vehicle Model TruckSim is a software tool developed by Mechanical Simulation Corporation for simulating and analyzing the dynamic behavior of medium to heavy trucks, buses and articulated vehicles. It is a globally preferred tool for vehicle dynamics, vehicle control development and test engineering. The vehicle model used in this simulation is a bus of category M 3 from TruckSim and vehicle specifications are shown in Table 1. Table 1. Vehicle Specifications Parameters Gross vehicle mass wheelbase Track width Effective rolling radius of tire Height of gravity center/mm Maximum braking pressure Maximum power Shifting control value 7690[kg] 4490[mm] 2030[mm] 510[mm] 1200[mm] 0.7[MPa] 175[KW] Auto shift and auto clutch Simulation and Analysis The simulation was carried out using Trucksim and Matlab/Simulink to verify the effectiveness of the proposed control systems. Table 2 shows the simulation conditions. Table 2. Simulation Conditions Parameters unladen Loaded condition Payload 0[kg] 2000[kg] Initial speed 70[km/h] Steering angle 0[ ] theoretical coefficient of adhesion 0.8 As we focus on the distribution of braking force in this paper, the simulations were performed under the assumption that the relationship between the brake pedal travel and the objective braking rate is linear. The vehicle starts braking evenly at 3s. The braking rate increases to 0.5 at 8s and then is held still.

5 Advanced Materials Research Vols Simulation under an unladen condition Fig. 2 Adhesion utilization curves during a normal braking Fig. 3 Adhesion utilization curves of mode one Fig. 2 shows the curves of the adhesion utilization of each axle and the objective and actual braking rates when the optimum braking force distribution control is employed. The adhesion utilization of each axle is equal to the braking rates, which means that both the optimum braking force distribution control and deceleration control are achieved under an unladen condition. Fig. 3 shows the simulation results when the front axle has greater wear. The braking force distributed to the rear axle is increased while the braking force to the front axle is released. The actual and objective braking rates are roughly the same. So the deceleration control is achieved. Fig. 4 Adhesion utilization curves of mode two Fig. 5 Braking distance Figure 4 shows the simulation results when the rear axle has greater wear. The braking force distributed to the front axle is increased while the braking force to the rear axle is released. The actual braking rate is roughly equal to the objective braking rate. So both the brake pad wear control and deceleration control are achieved under an unladen condition. Fig. 5 shows the braking distances which are 78.43m, 78.42m and 78.43m respectively. So the braking distance is independent from the wear conditions of the brake pads.

6 2104 Manufacturing Process and Equipment Simulation under a loaded condition Fig. 6 Adhesion utilization curves during a normal braking under a loaded condition Fig. 7 Adhesion utilization curves of mode one under a loaded condition Fig. 6 shows the curves of the adhesion utilization of each axle and the objective and actual braking rates when the optimum braking force distribution control is employed. The adhesion utilization of each axle is equal to the braking rates, which means that both the optimum braking force distribution control and deceleration control are achieved under a loaded condition. Fig. 7 shows the simulation results when the front axle has greater wear. The braking force distributed to the rear axle is increased while the braking force to the front axle is released. The actual and objective braking rates are roughly the same. So the deceleration control is achieved. Fig. 8 Adhesion utilization curves of mode Fig. 9 Braking distance under a loaded condition two under a loaded condition Fig. 8 shows the simulation results when the rear axle has greater wear. The braking force distributed to the front axle is increased while the braking force to the rear axle is released. The actual braking rate is roughly equal to the objective braking rate. So both the brake pad wear control and deceleration control are achieved under a loaded condition. Fig. 9 shows the braking distances which are 78.86m, 78.86m and78.87m respectively. The braking distance is independent from the wear conditions of the brake pads. By comparison with the simulation results shown in Fig. 5, we learn that the braking distance is roughly the same when using the same brake pedal operation, so the braking distance is independent from the load conditions. This will improve braking safety.

7 Advanced Materials Research Vols Conclusions A brake pad wear control algorithm used under uncritical braking conditions is proposed to reduce the difference in brake pad wear between the front and rear axles. Simulation analyses of braking using vehicle models with equal brake pad wear, greater wear on the front axle and greater wear on the rear axle were carried out respectively to confirm the effectiveness of the proposed controls. The results are shown as follows: (1) The braking force can be distributed according to the wear conditions of the brake pads to reduce the difference in brake pad wear between the front and rear axles when braking uncritically, while the deceleration control still functions properly. (2) The braking force distribution control can distribute the braking force according to the dynamic wheel loads to make sure that the adhesion utilization of each axle coincides with the braking rates under normal braking conditions, and the deceleration control makes the braking characteristics always constant under any load conditions. Acknowledgements This work was financially supported by the National Natural Science Foundation of China ( ) and ( ), China Postdoctoral Science Foundation ( ) and (2012T50291). References [1] Information on [2] Wenfa Luo. Commercial Vehicle (6),P:126~128. In Chinese [3] Karthikeyan P, Subramanian S.C.. Development and Modeling of an Electropneumatic Brake System. Intelligent Vehicles Symposium (2009), P: 858~863 [4] Zikai Liu, Hui Chen, Jianzong Yuan, et al. Modeling and Simulation for Electronic Braking System of Commercial Vehicles SAE-China Congress Proceedings(2008),P: 1339~1344. In Chinese [5] Liang Chu, Jie Liu, Jiayun Gu, et al. China Patent (2007). In Chinese [6] Jie Liu. Study on the Control Algorithm of Electronical Controlled Braking System for Commercial Vehicle. Jilin University, In Chinese [7] Changfu Zong, Wei Li, Hongyu Zheng. Automotive Engineering, 2011 (33), P: 885~889. In Chinese [8] Keliang Sun, Tongli Lu. Urban Vehicles, 2006 (5), P: 50~52. In Chinese [9] Zhisheng Yu. Theory of Automotive. China Machine Press In Chinese [10] MauriHaataja, TatuLeinonen. SAE [11] M.Nakazawa, O.Isobe, S.Takahashi, et al. Vehicle System Dynamics, 1995(24), P: 413~426 [12] UN ECE R13, Uniform Provisions Concerning The Approval of Vehicles of Categories M,N and O With Regard to Braking

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